Nuclear magnetic resonance (NMR) methods are well established in the laboratory to measure fluid flow in pipes, rocks, and biological systems. Typically, flow is measured by encoding molecular displacements in the phase of nuclear spins in the (flowing) fluid, during an evolution interval Δ. An encoding and decoding of the spin's position before and after interval Δ is affected by means of static or pulsed field gradients. The NMR signal is measured for either a range of phase encoding times or a range of gradient strengths, or both, and then analyzed. For small static or pulsed field gradient strengths or encoding times, the shift of the phase of the NMR signal is proportional to the velocity and the time which has elapsed between the encoding and the decoding steps. The complete probability distribution of molecular displacements during the evolution period Δ can be obtained from pulsed field gradient-NMR (PFG-NMR) by extending the measurements to larger pulsed field gradient strengths, then Fourier transforming the data. The PFG-NMR type of experiment has been called NMR-scattering in the published literature, and the extraction of probability distributions from such experiments is commonly referred to as a measurement of the propagator in the published literature.
NMR measurements are also commonly used in the borehole to probe the NMR decay behavior of the stationary fluid in the reservoir rock. In these techniques, magnetic fields are established in the formation using suitably arranged permanent magnets. These magnetic fields induce nuclear magnetization, which is flipped and otherwise manipulated using on-resonance radio frequency (RF) pulses. NMR echoes are observed, and their dependence (of their magnitude) on pulse parameters and on time is used to extract information about the formation and the fluids in it.
Fluids are routinely sampled in the borehole using various fluid samplers, such as Schlumberger's MDT™ module. The MDT™ tool includes at least one fluid sample bottle, a pump to extract the fluid from the formation, and a contact pad having a hole. When the device is positioned proximate a region of interest, the pad is pressed against the borehole wall, making a tight seal. Fluid in the formation is induced to flow, by pumping fluid out of the formation through the hole in the pad. When the operator is convinced that the fluid is ‘pure’ reservoir fluid (i.e., the sample includes acceptable levels of mud or other contaminants), a sample of the fluid is either analyzed in-situ or placed into a sample bottle for later analysis. The module is then moved to the next region of interest. Information regarding the movement of fluid in the formation during the pumping process (“pumpout”) can provide valuable information related to formation and fluid sample properties.
Fluid flow towards the borehole is also routinely produced during pressure testing, essentially in the same manner as described for the MDT™ tool described above. Accordingly, useful information may be similarly obtained during this process.
To date, there is no method or apparatus to obtain information regarding the fluid flow or the formation during these induced fluid flow scenarios. Accordingly, it is an object of the present invention to provide an NMR method to probe the flow which occurs inside the formation during fluid pumpout.